The sensing of gases is important to many endeavors such as environmental monitoring, patient care, anesthesiology, personal safety, and process control. The electronic gas sensors typically used in these applications feature transduction mechanisms that rely on surface interaction (e.g., absorption or adsorption) of an analyte gas. The amount of interaction, and hence the output signal from these sensors, generally is proportional to the partial pressure of the analyte gas in the gas sample contacted with the sensor.
More specifically, gas sensors are susceptible to noise that can affect the signal from the sensor. One type of drift noise, also called “flicker noise,” is characterized by slowly varying signals resulting from temperature variations and/or “aging” of the sensor.
Drift noise resulting from “aging” of the sensor often exhibits a magnitude that is. inversely proportional to the frequency at which “zero” measurement signals are taken using the sensor. In other words, drift noise≈1/f, where f is the frequency. As frequency is inversely proportional to the time between “zero” measurements (i.e., f=1/t, where t is the time), drift noise will be proportional to the time between zero measurements (i.e., drift noise≈1/(1/t) t). Where the measurement of gas concentrations is accomplished over a period of hours or days, the result will be very large measurement periods (t), very small frequency values (f), and correspondingly high values of 1/f. In summary, the longer the interval between zero adjustments of the sensor, the greater the amount of drift that is likely to be observed.
Conducting a “zero” measurement immediately followed by a “target” measurement could minimize drift by minimizing the interval between zero adjustments. But such approach has not been effective, because it creates another source of drift or noise when a pump is utilized to effect gas-flow to the sensor—i.e., pressure variation in the measurement zone at the sensor element. This can be seen from a more detailed analysis of the mechanisms at work. Thus, to secure a reference measurement an analyte gas and/or unwanted contaminants are, for example, removed by a trap (often referred to as a “scrubber”) through the use of physical and/or chemical means. A “zero” measurement is provided by exposing the sensor to the gas sample from the trap. Thereafter, a “target” (or normal) measurement is taken by exposing the sensor to a gas sample not previously contacted with the trap. Ideally, the subtraction of the “zero” measurement from the “target” measurement provides a signal that has not been corrupted by long-term drift, especially where the period between the “zero” and “target” measurements can be kept short. Systems designed to reduce the effects of sensor baseline drift “noise” on the measurement typically have a valve located before the sensor, allowing switching of the flow to the sensor between a trap (which provides the “zero” reference gas sample to the sensor) and a direct source providing the “target” gas sample. However, flow switching of this sort tends to result in pressure changes in the measurement zone at the sensor which themselves cause noise that detracts from the sensitivity and accuracy of detection.
In connection with the foregoing, a number of known fluid analysis systems utilize means for switching between two or more fluid sources. For example, the infrared gas analyzing apparatus of U.S. Pat. No. RE31,438 discloses a valve system connected to two analysis cells, wherein the flow from two sources can be alternated between the analysis cells. The infrared fluid analyzer of U.S. Pat. No. 4,902,896 (Fertig et al.) uses a single valve switch for alternating flow to the pump between a raw fluid source and a “calibration” fluid source. The gas analyzing apparatus of U.S. Pat. Nos. 5,332,901, 5,340,987, and 5,457,320 (Eckles et al.) features a gas valve for the provision of either a processed or unprocessed gas to the apparatus' infrared sensor. U.S. Pat. Nos. 6,397,660 and 6,550,308 (Kikuchi et al.) disclose a gas analyzing apparatus with a valve system for switching between an unprocessed gas sample and processed gas sample.
More specifically, while periodically shifting between a “zero” gas sample and “target” gas sample is effective in reducing drift noise due to temperature change or “aging,” conventional systems experience problems with sensor noise due to the pressure variations caused when the flow to the sensor is modulated from the “zero” reference source to the “target” source. Unfortunately, any variation in pressure at the gas sensor will result in a corresponding variation in the sensor signal. Thus, pressure variation will introduce an independent source of “noise” to the sensor signal output.
Some gas sensors rely on passive diffusion to bring the analyte gas to the sensor. In this case, the pressure of the total gas sample is the same as the ambient atmospheric pressure. Atmospheric pressure variations are limited in magnitude and usually occur slowly. It follows that sensors using passive gas sampling are not normally afflicted with serious problems of sample pressure induced variation in sensor output signals. But, for certain important applications this is not a real world answer, as most high performance gas detection and measurement systems require a sampling pump to bring known quantities of representative gas samples to the sensing element quickly. Pumps generate a significant pressure differential to cause gas flow, and abrupt switching from one pressurized gas stream to another can cause a pressure variation. If the gas pressure at the sensor varies then the sensor signal will vary. Pump pressure variations, therefore, introduce a source of noise to the sensor signal output. Even though ordinarily for high precision gas sensing, the pumps utilized cause no more than modest pressure variation, this still introduces a disadvantageous amount of additional noise. And, of course, the high performance of more substantial pumps is sacrificed.
In any event, the problem with pressure variations is much more pronounced in systems where it is desired to use a valve ahead of the sensor to switch the sensor flow between a scrubber trap (to provide a “zero” reference gas to the sensor by removing unwanted contaminants from the gas sample) and the direct source of the gas sample. A typical system of this sort is illustrated in FIG. 1. The purpose of such a system is to reduce the effects of sensor baseline drift “noise” on the measurement.
A strategy to remedy this problem is to match the flow restriction imposed by the direct flow path to that of the trap. While this strategy may offer some improvement, it still is not adequately effective because the pressure of the gas present at the connection between the trap and the valve (or, alternately, between the direct sample path and the valve) changes drastically whenever the valve is switched to the other path and the flow to the pump vacuum is interrupted. This change in pressure at the measurement situs introduces “noise” and/or other inaccuracy.
The development of a simple method and apparatus for analyzing gas samples, which minimize the effects of noise and drift, by decreasing the interval between “zero” and target measurements and also providing the ability to alternate the flow to the sensor between a reference gas source and target gas source while maintaining a substantially constant pressure at the sensor, would be a significant step forward in the art.